Chemical-mechanical processing (CMP) has been used by the semiconductor industry for a number of chip development generations to ensure planar surfaces during processing. As semiconductor chips and chip processing have become more complex and through time a greater consideration various CMP processes have been developed and used. A CMP process is used over and over again for almost every layer of semiconductor production. Different layers and steps of the overall process have different needs as the materials used differ.
One of the ways to decrease the overall amount of time it takes to process a semiconductor chip is to reduce the number of steps involved in the CMP process, as it is a processing step that occurs over and over again. A number of parties have patented CMP methods that have minimized the number of steps involved with CMP at different points in the overall chip manufacture. One point during the process where CMP is done is during the early stages of chip manufacture when the transistor levels are being formed. As the size of the transistors shrinks and different components like shallow trench isolation (STI) are added, changes in the CMP process have been contemplated. Traditionally, when developing new CMP schemes, developers have tried to continue to use traditional CMP slurries and endpoint detection methods as it reduces the number of factors which must be changed to achieve a viable process change.
When throughput becomes a process engineer's overriding concern the new process steps may require different materials in the form of CMP slurries and/or endpoint detection methods. Several new CMP techniques have been developed that have required the use of new types of CMP slurries. A number of new and different slurry compositions exist. Most slurries can be grouped by families according to the chemical composition and number of components. Each family of slurry tends to have enough physical and chemical properties in common that a common preferable endpoint detection method can be developed. Currently, there is a class of slurries that are called high viscosity slurry (HVS) that contain detectable amounts of ammonium ions prior to processing. Most known endpoint detection methods based on the detection of NH4+/NH3 are not advantageous from a manufacturing standpoint when HVS is used.
Another slurry under development does not use ammonium but it has a pH that is slightly acidic and for that reason the CMP process products are not converted into a chemical compound that is easily detectable using current endpoint detection methods.
Integration of a new endpoint detection method into current generation CMP equipment should also be a consideration. CMP equipment tends to be an integral part of a semiconductor fabrication system. Fabrication equipment placement is a design decision and any endpoint detection scheme that requires additional substantial equipment enhancement would be impractical, if not financially unfeasible in a manufacturing environment. One reason that it would be financially unfeasible to add apparatus have a substantial footprint is that it would require that most of the equipment required to create a semiconductor chip be reorganized. By footprint it is meant the amount of space an apparatus takes up by linear foot.
In current technology endpoint detection methods the slurry to be tested is usually collected from the active pad area. An example of a current technology CMP apparatus is shown in FIG. 5. The underside of workpiece 100 (in this case, as semiconductor wafer) is being polished on a polishing pad 102 in the presence of a slurry. Note that the workpiece is typically held by some sort of carrier but the carrier is not shown for the sake of clarity. As shown, pad 102 rotates as shown by arrow 104. Wafer 100 also optimally rotates as indicated by arrow 106. Slurry is dispensed to pad 102 upstream or in front of wafer 100 and generally moves as indicated by arrows 108 downstream from or behind (i.e. clockwise from) wafer 100. Slurry is usually directly contacted at point 110 and the amount of ammonia in the gas sampled. When the slurry is acidic and ammonium is present instead of ammonia gas then the ammonia gas can not be directly sampled in gas form from point 110.
Thus there remains a need for a CMP endpoint detection apparatus that is capable of detecting endpoint in chemistries that are not compatible with current endpoint detection methods. There also remains a need for a CMP endpoint detection apparatus that has substantially the same footprint as current generation CMP ammonia gas endpoint dection apparatus.